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Plumbene on a Magnetic Substrate: a Combined STM and DFT Study

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Plumbene on a Magnetic Substrate: a Combined STM and DFT Study Plumbene on a magnetic substrate: a combined STM and DFT study Gustav Bihlmayer,1, ∗ Jonas Sassmannshausen,2 Andr´eKubetzka,2 Stefan Bl¨ugel,1 Kirsten von Bergmann,2, y and Roland Wiesendanger2 1Peter Gr¨unberg Institut and Institute for Advanced Simulation, Forschungszentrum J¨ulich& JARA, 52425 J¨ulich, Germany 2Department of Physics, University of Hamburg, D-20355 Hamburg, Germany (Dated: June 26, 2020) As heavy analog of graphene, plumbene is a two-dimensional material with strong spin-orbit coupling effects. Using scanning tunneling microscopy (STM), we observe that Pb forms a flat hon- eycomb lattice on an Fe monolayer on Ir(111). In contrast, without the Fe layer, a c(2×4) structure of Pb on Ir(111) is found. We use density functional theory (DFT) calculations to rationalize these findings and analyze the impact of the hybridization on the plumbene band structure. In the unoc- cupied states the splitting of the Dirac cone by spin-orbit interaction is clearly observed while in the occupied states of the freestanding plumbene we find a band inversion that leads to the formation of a topologically non-trivial gap. Exchange splitting as mediated by the strong hybridization with the Fe layer drives a quantum spin Hall to quantum anomalous Hall state transition. Since the exotic properties of graphene have been ture but without band inversion near the Fermi level [13]. discovered about 15 years ago [1], the field of two- Electronically it is similar to a Bi(111) bilayer with less dimensional materials in general and honeycomb struc- electrons (ZBi = 83, ZPb = 82). DFT studies suggested tures in particular has seen a dramatic increase in pop- that doping or chemical modification of plumbene [14] ularity [2]. Topological properties in these lattices de- might be necessary to achieve topological effects. Maybe pend significantly on the strength of spin-orbit coupling it is because of these findings that the quest for plumbene (SOC) effects that are notoriously small in graphene, es- has not really started yet. pecially at the K point [3]. Therefore, although the quan- Using scanning tunneling microscopy (STM) and DFT tum spin Hall effect (QSHE) was first theoretically pre- we show in this Letter that (i) using an appropriate sub- dicted for graphene [4], experimentally it was first ver- strate it is possible to form a flat plumbene lattice and (ii) ified in materials containing heavy elements like HgTe that the electronic properties of ’flat plumbene' are rather quantum wells [5]. Since then, numerous studies have fo- exciting: Calculations predict a band-inversion in the va- cused on the synthesis and properties of heavier analogs lence bands that leads to topologically protected edge of graphene like silicene [6], germanene [7] or stanene states. Further (iii), on the ferromagnetic substrate that (Sn) [8]. But the formation of double bonds in this se- enables the formation of plumbene the induced exchange ries seems to be restricted to the carbon-based material splitting drives this feature into a quantum anomalous only and freestanding heavier analogs are considered un- Hall gap. Such exchange coupling opens the way to real- likely to form [9]. Consequently, the first silicene was ize a quantum Hall effect without external magnetic field, reported as adlayer on Ag(111) [10] and it is still challeng- a phenomenon envisioned theoretically in the eighties [15] ing to balance the interaction with the substrate required and only recently realized experimentally [16] at very low for formation with the electronic independence neces- temperatures. sary to study the topological properties via electronic We have deposited sub-monolayer amounts of Pb onto transport effects [6]. Furthermore, all heavier analogs a sample with extended Fe monolayer areas on an Ir(111) of graphene have a tendency to pronounced buckling of single crystal surface, see overview STM image in Fig. 1. their honeycomb structures, resulting in severe changes Because we observed severe intermixing of Pb and Fe of the electronic properties as compared to the ideal flat for Pb growth at room temperature we have cooled the structures [11]. Therefore, it came recently as a welcome Fe/Ir(111) to about 140 K prior to the Pb deposition [17], surprise that stanene was observed to grow on Cu(111) which results in large and well-ordered patches of Pb as a flat honeycomb lattice [12]. Despite the metallic both on the bare Ir(111) and the Fe-covered Ir(111), see arXiv:2006.14516v1 [cond-mat.mtrl-sci] 25 Jun 2020 substrate, a topological edge state could be observed on labels for the different layers in Fig. 1. The Fe mono- these islands - although 1.3 eV below the Fermi level. layer grows pseudomorphically in fcc stacking on the In this quest for heavy honeycomb structures the Pb Ir(111) substrate and in this spin-polarized STM mea- analog, plumbene, appeared relatively late on the scien- surement [18] the observed roughly square superstruc- tific stage. Isoelectronic in its valence shell with C, Si, ture with a periodicity of about 1 nm originates from the Ge and Sn it is the heaviest graphene analog and ex- magnetic nanoskyrmion lattice [19]. No magnetic signal pected to show the most pronounced SOC effects [11]. has been observed on the Pb monolayers. Density functional theory (DFT) studies of plumbene A closer view on the Pb deposited directly on the Ir and predicted the formation of a buckled honeycomb struc- on Fe/Ir is shown in Figs. 2(a) and (b), respectively. Here 2 FIG. 1. Pseudo 3-dimensional STM image of a sample of Pb FIG. 2. Closer view constant-current STM images of the on Fe on Ir(111), the different exposed surfaces are labeled. Pb monolayer on (a) the bare Ir(111) surface and (b) the Fe Pb was deposited well below room temperature and grows as monolayer on Ir(111). Both the symmetry and the atomic dis- monolayer high ordered islands both on the bare Ir surface tances for the two differently ordered Pb monolayers are dif- and on the Fe monolayer on Ir. The superstructure on the ferent and the corresponding structure models are presented pseudomorphic Fe monolayer on Ir is not due to the structure in (c) and (d). The white dashed rectangle in (c) refers to but corresponds to the magnetic signal of the nanoskyrmion the primitive unit cell of the c(2 × 4) structure and the white lattice. Measurement parameters: U = +5 mV, I = 3 nA, dashed diamond in (d) marks the p(2 × 2) unit cell; blue T = 4 K, Cr-bulk tip. dashed circles mark positions that are empty for a lower atom- density configuration and occupied for a higher-atom density configuration (confer text). Measurement parameters: (a) the superstructures are of structural origin. Due to the U = +10 mV, I = 1:5 nA; (b) U = +5 mV, I = 2 nA; both large lattice mismatch of Pb and Ir, the Pb overlayers are T = 4 K, Cr-bulk tip. Information on the bias dependence is not pseudomorphic, but instead form layers with reduced provided in the supplementary information, Fig. S1. atom density. Nevertheless, the Pb superstructures are commensurate with the substrate and atoms reside at specific adsorption sites of the Ir or Fe/Ir, an indication structural unit cell of the honeycomb has lattice vec- ˚ that the Pb-substrate interaction is comparable to the ptors with a length of 5.43 A, the Pb-Pb distance is only ˚ Pb-Pb interaction. 6=3aIr =p 3:13 A, i.e. more than 10% shorter than in fcc On Ir(111) we find a c(4 × 2) Pb overlayer that can be Pb (aPb= 2 = 3:52 A).˚ Comparing graphene with dia- described with a rectangular unit cell, see white dashed mond a similar contraction of bond distances (1.42 A˚ vs. rectangle in Fig. 2(c), like in the graphene intercalated 1.55 A)˚ can be observed, suggesting that the Pb-Pb bond Pb films on Ir [20]. The Pb atoms all occupy the same is modified in a similar way as the C-C bond for the two adsorption sites and the Pb-Pb distances are 4.70 A˚ and different allotropes. 5.43 A˚ for the two orthogonal directions. For symmetry Using density functional theory we study the energet- reasons three rotational domains are found on a larger ics of different Pb monolayers on Ir(111) and Fe/Ir(111). scale. Based on the experimental findings we compare four pos- In contrast, on the Pb/Fe/Ir we find a honeycomb ar- sible arrangements of Pb at these surfaces: p(2 × 2) and rangement of the Pb atoms, see experimental data and c(2 × 4) unit cells (uc.) with one or two Pb atoms per corresponding structure model in Figs. 2(b),(d), i.e. the cell, i.e. the structures shown in Figs. 2(c) and (d) with Pb grows on Fe/Ir(111) in the modification of plumbene. and without the atoms at positions marked by the dashed The structural p(2 × 2) unit cell (see white dashed di- blue circle. amond) contains two Pb atoms that adsorb in a fcc Comparing fcc and hcp adsorption sites, a single Pb and a hcp site. The Pb atom density of this honey- atom on Ir(111) forming a p(2 × 2) or c(2 × 4) structure comb plumbene layer is twice that of the c(4 × 2) Pb always prefers the fcc site by 49 meV/Pb. The p(2 × 2) overlayer on Ir. Since both the Fe/Ir and the Ir(111) and c(2 × 4) arrangements differ by only 5 meV in favor surface have the same symmetry and identical atomic of the former.
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